Proper wheel alignment is necessary for safe vehicle operation. Wheels that are out of alignment, in relation to each other, cause their respective tires to wear unevenly. This gives the vehicle an uncomfortable ride, and increases the chance of an accident due to tire failure or improper steering response.
Vehicle wheels are fixed into proper alignment at the time of manufacture. Subsequent consumer use often involves the wheels contacting uneven road surfaces, or inadvertently striking objects and curbs, which can shift the proper alignment settings.
There are three types of wheel alignment settings, that can cause driving difficulty for the vehicle operator if they shift. The first setting is the "camber", or the side to side angle of the wheel from a vertical line. The second setting is "caster", which is the forward or rearward angle of a line, from a vertical line, as measured between the upper and lower ball joints of the wheel assembly. The third setting is the line of travel that a single wheel wishes to follow as it rotates, or what is commonly referred to as toe in or toe out settings of a wheel, in relation to the other wheel on a given axle. Wheels on a single axle, that do not have the proper toe in or toe out setting, can cause the vehicle to pull or turn from one side to the other, when travel is attempted along a straight line.
Various methods and apparatus have been used to determine and correct these three settings. Wheels on each axle can be compared to each other to determine toe in or toe out, and leveling systems are used to check the camber and caster. The methods previously used have required large and expensive systems, with a great deal of training and expertise needed to operate properly. Current state of the art equipment comprise electronic measuring devices, that determine the alignment of wheels, in which the average mechanic has difficulty operating without extensive training. The cost of this equipment often exceeds the ability of a small shop owner's financial means. In addition to the electronic equipment, a large lift is necessary, since the entire vehicle must be lifted off of the ground during the alignment process.
The camber and castor settings can be adjusted on some vehicles, while others vehicles are not adjustable, due to the manufacturer's design. Vehicle manufacturers have different tolerances for camber and castor, depending on the vehicle. While proper camber settings are generally designated as zero degrees from a vertical line, proper castor settings may vary over several degrees forward or behind a vertical line, depending on the vehicle.
Toe in or toe out depends on the type of vehicle, and intended use. Nearly every vehicle has adjustments available on the wheel assembly, to modify the toe in or toe out settings. Fixed rear wheels may have adjustments made using such methods as the insertion of shims.